This invention relates to data transmission systems of the kind that allow a number of devices to communicate with one another over a network of transmission links. Such networks can be classified into a number of different types, depending on the way the links are arranged. This invention is concerned in particular with the type in which the links are joined end to end to provide a single continuous path for the flow of data, for example round a ring or loop.
Commonly each device is connected to such a network through an interface unit which transfers information to the device from the network or vice versa. The interface units are connected to one another by the individual transmission links of the network. It is normally necessary for each interface unit to derive a clock signal from the information it receives on its incoming link and use it to control the transmission of information onto its outgoing link. Any such derived clock signal is liable to jitter and with a sequential arrangement of interface units such as has been described this jitter accumulates to give rise to timing difficulties which can limit the number of interface units it is possible to interconnect. Timing variations can also arise from changes in the physical lengths of the transmission links, caused for example by changes in temperature.